1. Field of the Invention
The present invention relates to an EL (electro-luminescence) display (an electro-optical device) formed by preparing an EL element on a substrate. More particularly, the invention relates to an EL display using a semiconductor element (an element using a semiconductor thin film). Furthermore, the present invention relates to an electronic equipment in which the EL display is used in a display portion thereof.
2. Description of the Related Art
In recent years, technology for forming a TFT on a substrate has been largely improved, and an application development of the TFT to an active matrix type display device has been carried out. In particular, the TFT using a polysilicon film has a higher electric field effect mobility than the TFT using a conventional amorphous silicon film, and therefore, the TFT may be operated at a high speed. Thus, the pixel control which has been conducted at a driver circuit outside of the substrate may be conducted at the driver circuit which is formed on the same substrate as the pixel.
Such an active matrix type display device can, by preparing various circuits and elements on the same substrate, obtain various advantages such as a decrease in the manufacturing cost, a decrease in the size of the display device, an increase in the yield, and a decrease in the throughput.
Further, research on the active matrix type EL display having an EL element as a self-light-emitting device is becoming more and more active. The EL display is referred to as an organic EL display (OLED) or an organic light-emitting diode (OLED).
The EL display is a self-light-emitting type unlike a liquid crystal display device. The EL element is constituted in such a manner that an EL layer is sandwiched between a pair of electrodes. However, the EL layer normally has a lamination structure. Typically, the lamination structure of a “positive hole transport layer/a luminous layer/an electron transport layer” proposed by Tang et al. of the Eastman Kodak Company can be cited. This structure has a very high light-emitting efficiency, and this structure is adopted in almost all the EL displays which are currently subjected to research and development.
In addition, the structure may be such that on the pixel electrode, a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer, or a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer/an electron injection layer may be laminated in order. Phosphorescent dye or the like may be doped into the luminous layer.
In this specification, all the layers provided between the pixel electrode and an opposite electrode are generally referred to as EL layers. Consequently, the positive hole injection layer, the positive hole transport layer, the luminous layer, the electron transport layer, the electron injection layer and the like are all included in the EL layers.
Then, a predetermined voltage is applied to the EL layer having the above structure from the pair of the electrodes, so that a recombination of carriers is generated in the luminous layer and light is emitted. Incidentally, in this specification, the fact that the EL element is emitted is described as the fact that the EL element is driven. Furthermore, in this specification, the light-emitting element formed of the anode, the EL layer and the cathode is referred to as an EL element.
An analog type driver method (analog drive) can be given as a method of driving an EL display. An analog drive EL display is explained using FIGS. 18 and 19.
The structure of a pixel portion of the analog drive EL display is shown in FIG. 18. Y gate signal lines (G1 to Gy) for inputting gate signals are connected to gate electrodes of switching TFTs 1801 of pixels. One of a source region and a drain region of the switching TFT 1801 of each pixel is connected to x source signal lines (also referred to as data signal lines) (S1 to Sx) for inputting analog video signals, and the other is connected to the gate electrode of an EL driver TFT 1804 of each pixel and to a capacitor 1808 of each pixel.
The source region and the drain region of the EL driver TFT 1804 incorporated in each of the pixels is connected to the power source supply lines (V1 to Vx) while the other is connected to the EL element 1806. The potential of the power source supply lines (V1 to Vx) is referred to as the potential of the power source. Note that, the power source supply lines (V1 to Vx) is connected to a capacitor 1808 incorporated in each of the pixels.
The EL element 1806 comprises an anode, a cathode and an EL layer provided between the anode and the cathode. In the case where the anode is connected to the source region or the drain region of the EL driver TFT 1804, namely, in the case where the anode is the pixel electrode, the cathode which is the opposite electrode is held at a constant potential. On the contrary, in the case where the cathode is connected to the source region or the drain region of the EL driver TFT 1804, that is, in the case the cathode is the pixel electrode, the anode, which is an opposite electrode is held at a constant potential.
The opposite electrodes are normally maintained at a constant electric potential, and in the present specification, the electric potential of the opposite electrodes is referred to as a steady-state electric potential. Note that an power source for imparting the steady-state electric potential to the opposite electrodes is referred to as a steady-state power source. The electric potential difference between the steady-state electric potential of the opposite electrodes and the power source electric potential of the pixel electrodes is an EL driver voltage, and the EL driver voltage is applied to the EL layers.
A timing chart for a case of driving the EL display by the analog method is shown in FIG. 19. A period during which one gate signal line is selected is referred to as one line period (L). Further, a period until selection of all the gate signal lines (G1 to Gy) is completed corresponds to one frame period (F). There are y gate signal lines for the case of the EL display of FIG. 18, and therefore y line periods (L1 to Ly) are formed during one frame period.
Note that 60 or more frame periods are formed during one second in the EL display drive. In other words, 60 or more images are displayed during one second. If the number of images displayed in one second becomes less than 60, then problems such as image flicker start to become visually conspicuous.
The number of line periods during one frame period increases as the number of gradations increases, and the driver circuit must operate at a high frequency.
First, electric power source supply lines (V1 to Vx) are maintained in an off-power source electric potential. Note that the off-power source electric potential in an analog driver method is in a range in which the EL elements do not emit light and is the same strength as the steady-state electric potential. Note also that the difference between the off-power source electric potential and the steady-state electric potential is referred to as an off EL driver voltage. Ideally, it is preferable that the off EL driver voltage be 0 V, but it is acceptable provided that it is such that EL elements 1806 do not emit light.
A gate signal is input to the gate signal line G1 in the first line period (L1). An analog video signal is then input to the source signal lines (S1 to Sx) in order. A switching TFT (1,1) is therefore in an On state (on), and consequently the analog video signal input to the source signal line S1 is input to the gate electrode of an EL driver TFT (1,1), through the switching TFT (1,1).
The electric potential of the power source supply line V1 then changes from the off-power source electric potential to a saturation power source electric potential. Note that, throughout this specification, saturation power source electric potential refers to an electric potential having an electric potential difference with the steady-state electric potential to the extent that the EL element emits light. Note also that this electric potential difference is referred to as a saturation power source voltage.
When the analog video signal is input to the gate electrode of the EL driver TFT and one of the source region and the drain region is maintained at the saturation power source electric potential, the other becomes the on-power source electric potential. Note that the difference between the on EL driver electric potential and the steady-state electric potential is referred to as an on EL driver voltage. Further, the on EL driver voltage and the off EL driver voltage are generically referred to as an EL driver voltage throughout this specification.
The on driver voltage is then applied to the EL element and the pixel performs display. The amount of electric current flowing in channel forming regions of the EL driver TFTs is controlled by the voltage size of the analog video signal input to the gate electrodes of the EL driver TFTs. The size of the on EL driver electric potential is therefore controlled by the analog video signal applied to the gate electrode of the EL driver TFT (1,1). Consequently, the size of the on EL driver voltage applied to the EL element is also controlled by the analog video signal applied to the gate electrode of the EL driver TFT (1,1).
Next, the analog video signal is similarly applied to the source signal line S2, and a switching TFT (2,1) turns on. The analog video signal input to the source signal line S2 is therefore input to the gate electrode of the EL driver TFT (2,1) through the switching TFT (2,1).
The EL driver TFT (2,1) is therefore placed in the On state. The electric potential of the power source supply line V2 then changes from the off-power source electric potential to the saturation power source electric potential. The on driver voltage, whose size is controlled by the analog video signal input to the gate electrode of the EL driver TFT (2,1), is therefore applied to the EL element, and the pixel performs display.
By repeating the above operations and completing input of the analog video signal to the source signal lines (S1 to Sx), the first line period (L1) is completed. The second line period (L2) begins next, and the gate signal is input to the gate signal line G2. Then, similar to the first line period (L1), the analog video signal is input to the source signal lines (S1 to Sx) in order.
The analog video signal is input to the source signal line S1. A switching TFT (1,2) turns on, and therefore the analog video signal input to the source signal line S1 is input to the gate electrode of an EL driver TFT (1,2) through the switching TFT (1,2).
The EL driver TFT (1,2) therefore turns on. The electric potential of the power source supply line V1 then changes from the off-power source electric potential to the saturation power source electric potential. The on driver voltage, whose size is controlled by the analog video signal applied to the gate electrode of the EL driver TFT (1,2), is therefore applied to the EL element, and the pixel performs display.
By repeating the above operations and completing input of the analog video signal to the source signal lines (S1 to Sx), the second line period (L2) is completed. The third line period (L3) begins next, and the gate signal is input to the gate signal line G3.
The above operations are then repeated in order, the gate signal is completely input to the gate signal lines (G1 to Gy), and all of the line periods (L1 to Ly) are completed. When all of the line periods (L1 to Ly) are complete, one frame period is complete. All of the pixels perform display during one frame period, forming one image.
Thus the amount of light emitted by the EL elements is controlled in accordance with the analog video signal, and gray scale display is performed by controlling the amount of light emitted. This method is a driver method referred to as the analog driver method, and gray scale display is performed by changing the amplitude of the signal.
A detailed description of the state of controlling the amount of electric current supplied to the EL elements by the gate voltage of the EL driver TFTs is made using FIGS. 3A and 3B.
FIG. 3A is a graph showing the transistor characteristics of the EL driver TFTs, and reference numeral 401 is referred to as an Id-Vg characteristic (also referred to as an Id-Vg curve). Id is a drain current, and Vg is a gate voltage here. The amount of electric current flowing with respect to an arbitrary gate voltage can be found from this graph.
A region of the Id-Vg characteristic shown by a dotted line 402 is usually used in driving the EL elements. An enlarged diagram of the region enclosed by the dotted line 402 is shown in FIG. 3B.
The shaded region in FIG. 3B is referred to as a sub-threshold region. In practice, this indicates a gate voltage in the vicinity of, or below, the threshold voltage (Vth), and in this region, the drain current changes exponentially with respect to the changes in the gate voltage. Electric current control is performed in accordance with the gate voltage by using this region.
The switching TFT turns on, and the analog video signal input within the pixel becomes the gate voltage of the EL driver TFT. At this point, the gate voltage and the drain current vary linearly in accordance with the Id-Vg characteristic shown in FIG. 3A. In other words, the drain region electric potential (the on EL driver electric potential) is determined in correspondence with the voltage of the analog video signal input to the gate electrode of the EL driver TFT, a predetermined drain current flows in the EL element, and the EL element emits light with an emission amount corresponding to the amount of electric current.
The amount of light emitted by the EL element is thus controlled in accordance with the video signal, and gray scale display is performed by the control of the amount of light emitted.
However, the above analog drive has a drawback in that it is extremely weak with respect to variations in the TFT characteristics. For example, suppose that the Id-Vg characteristic of a switching TFT differs from that of the switching TFT of an adjacent pixel displaying the same gradation (a case of an overall positive or negative shift).
In this case, the drain current of each switching TFTs becomes different on the degree of the variation, and a different gate voltage becomes applied to the EL driver TFT of each pixels. In other words, a different electric current flows in each of the EL elements, and as a result, the amount of light emitted differs, and display of the same gradation can not be performed.
Further, even supposing that equal gate voltages are applied to the EL driver TFT of each pixels, if there is dispersion in the Id-Vg characteristic of the EL driver TFTs, then the same drain current cannot be output. In addition, as is made clear from FIG. 3A, the region used is one in which the drain current changes exponentially with respect to changes in the gate voltage, and therefore even if the Id-Vg characteristic deviates by a slight amount, a situation can develop in which the amount of electric current output differs greatly even with equal gate voltages. If this occurs, then even if the same voltage signals are input, the amount of light emitted by EL elements in adjacent pixels differs greatly due to a slight deviation in the Id-Vg characteristic.
In practice, there is a multiplier effect between the variations in the switching TFTs and the EL driver TFTs, and therefore it becomes conditionally more difficult. The analog drive is thus extremely sensitive with respect to dispersion in TFT characteristics, and this disturbs the multi-colorization of conventional active matrix EL display devices.